Radiation therapy is the treatment of diseases, especially the treatment of tumors, including malignant tumors, with radiation. In radiation therapy, the ultimate aim is to destroy the malignant tissue without causing excessive radiation damage to nearby healthy, and possibly vital, tissue. This is difficult to accomplish because of the proximity of malignant tissue to healthy tissue.
Medical personnel and investigators have developed methods for preferentially irradiating deep seated diseased tissue as opposed to healthy tissue. These methods include the use of high energy X-ray beams together with cross fire and rotational techniques which create a radiation pattern that is maximized at the site of the diseased tissue. Nonetheless, some absorption and damage inevitably occurs to healthy tissue in the path through which radiation passes to arrive at deep seated diseased tissue.
One method of limiting the zone of irradiation utilizes radioactive articles in the form of small, radioactive "seeds," which are permanently implanted at the zone to be irradiated. Such seeds contain a radioactive source disposed within a sealed capsule. The seeds are injected or implanted into body tissue at the site to be treated. The small size of therapeutic seeds allows the seeds to be inserted within the tissue to be treated, in order to totally surround the tissue.
The advantages of interstitial implantation of a radiation-emitting article for localized tumor treatment have long been recognized. Interstitially implanted articles concentrate the radiation at a zone where radiation treatment is needed, i.e., near or within a tumor in order to directly affect surrounding tumor tissue, while exposing normal, healthy tissue to substantially less radiation than beaming radiation into the body from an external source.
Implanting radioactive articles directly into solid tumors to destroy the tumors is a therapy referred to as brachytherapy (i.e., short-range therapy). This form of therapy permits the application of larger doses of radiation directly to the tumor.
Radioactive seeds are disclosed, for example, in Lawrence U.S. Pat. No. 3,351,049 and Kubiatowicz U.S. Pat. No. 4,323,055. The seeds comprise a tiny sealed capsule having an elongate cavity containing the radioisotope, e.g., iodine-125 or palladium-103, adsorbed onto a carrier body. Because of the low energy X-rays emitted by iodine-125 and the short half-life of iodine-125, the seeds can remain implanted in the tissue of a patient indefinitely without excessive damage to surrounding healthy tissue or excessive exposure to other individuals near the patient.
In order to function effectively, the radiation emitted from the radioisotope within the seed cannot be blocked or otherwise unduly attenuated. Preferably, radiation emitted from the radioisotope is uniformly distributed from the seed in all directions, i.e., has an isotropic radial distribution. In particular, it is generally desirable to avoid seeds having end constructions having a greater concentration of radiation-absorbing material, which attenuates the therapeutic radiation required for the successful treatment of diseased tissue.
Providing a uniform distribution of radiation from a seed has been difficult to impossible to accomplish. For example, present-day seeds have a radioisotope adsorbed onto a carrier substrate, which is placed into a metal casing that is welded at the ends. The most advantageous materials of construction for the casing which encapsulates the radioisotope-laden carrier are stainless steel, titanium, and other low atomic number metals. However, problems exist with respect to sealing casings made from these materials. Such metallic casings typically are sealed by welding, but welding of such small casings is difficult because welding can locally increase the casing wall thickness, or can introduce higher atomic number materials at the ends of the casing where the welds are located. The presence of such localized anomalies can significantly alter the geometrical configuration at the welded ends, resulting in undesirable shadow effects in the radiation pattern emanating from the seed. Such seeds also have the disadvantage of providing a nonhomogeneous radiation dose to the target due to their construction, i.e., the relatively thick ends attenuate the radiation more than the relatively thin body of the seed.
Other methods of forming the seed casing include drilling a metallic block to form a casing, and plugging the casing to form a seal. However, this method suffers from the disadvantage that a casing of uniform wall thickness is difficult to obtain, and the radiation source, therefore, is not able to uniformly distribute radiation.
Several patents are directed to implantable radioactive seeds for use in brachytherapy. Examples of such patents include Kubiatowicz U.S. Pat. No. 4,323,055; Suthanthiran U.S. Pat. No. 4,891,165; Russell, Jr. et al. U.S. Pat. No. 4,784,116; Lawrence U.S. Pat. No. 3,351,049; Good U.S. Pat. No. 5,342,283; and Langton et al. U.S. Pat. No. 5,460,592. Although these patents illustrate improvements in construction of seeds for use in brachytherapy, the art seeds still suffers from the problem of providing a seed that, simultaneously, (a) is easy to manufacture, (b) provides adequate protection against leakage of radioactivity, and, importantly, (c) provides a uniform radiation dose in all directions. Significant advances have been made with respect to ease of seed manufacture and protection against leakage of radioactivity from the seed. The present invention is directed to providing a brachytherapy seed having these two attributes, and the additional attribute of providing a more uniform dose of radiation in all directions.